The present invention relates to a negative electrode current collector of an alkaline dry cell and more particularly, it relates to a negative electrode current collector capable of inhibiting generation of hydrogen gas and providing an improved liquid-leakage resistance for an alkaline dry cell using a mercury-free zinc alloy powder as a negative electrode active material and which use the negative electrode current collector of the present invention.
In alkaline dry cells using zinc as a negative electrode active material, hydrogen gas is generated owing to corrosion reaction of zinc during storage of the cells, resulting in increase of internal pressure of the cells to cause ejection of the electrolyte from the cells and thus, the leakage resistance of the cells decreases. In some cases, bursting of the cells may be brought about.
As a countermeasure against these problems, it has been generally conducted to use an amalgamated zinc powder containing mercury as a negative electrode active material in order to increase hydrogen overvoltage of zinc as a negative electrode active material to thereby inhibit corrosion of zinc and generation of hydrogen gas in the cells. As a current collector for the zinc negative electrode, copper or copper alloys have hitherto been generally used, and the surface of the current collector is amalgamated upon contacting with the amalgamated zinc negative electrode. Furthermore, Japanese Patent Kokaii (Laid-Open) Nos. 58-155653 and 61-56285 propose to remove impurities, especially, Fe, Ni, Cr, Co, Mo and W and oxides thereof, from the surface of the current collector, which promote corrosion of zinc, and to cleanse or polish the surface of the current collector with an alkali degreasing solution or chemical polishing solutions such as acids and hydrogen peroxide before fabrication of the cells to inhibit generation of hydrogen gas.
However, even when the current collector subjected to the cleansing or chemical polishing is used, alkaline dry cells which are excellent in corrosion resistance, reduced in fluctuation of quality and highly stabilized in quality have not yet been obtained with a zinc alloy powder free from mercury as the negative electrode active material. This is for the following reasons.
As materials for the negative electrode current collectors of alkaline dry cells, copper or copper alloys such as brass are generally used. These materials are drawn through several wire making steps as shown in FIG. 1 attached hereto to have a desired diameter. Molds used for the drawing are generally made of cemented carbide steels or hot tool steels and hence, the surface of the current collectors made of copper or copper alloys has fine fragments of a metal such as iron and nickel adhered thereto. In many cases, the fine fragments bite into and stick to the surface.
Therefore, the fine fragments cannot completely be removed by a conventional alkali degreasing treatment and may cause generation of hydrogen gas after fabrication of the cells.
Furthermore, even the polishing by an acid or chemical polishing agent which has a more effective cleansing action cannot completely remove the fine fragments which deeply bite into and stick to the surface of current collector, and the remaining fragments cause generation of hydrogen gas.